Thermal IR detectors are heated by the incident IR radiation and provide detection through the change in a measurable parameter. For these types of detectors, wavelengths of interest are mainly in the atmospheric windows—ranging from 3 to 5 (MWIR) and 8 to 14 (LWIR) μm wavelength ranges, due to the high transmission through atmospheric air of more than 80% and peak IR emission of room temperature bodies is at 9-10 μm of wavelengths. Thermal detectors like microbolometers are being used contact-less temperature measurement, night vision cameras for defense, security and surveillance applications, search and rescue and many other thermal imaging applications because of their low-cost, better performance and compact size. A main factor in dictating how well a thermal detector will work is the detector's responsivity. Responsivity is the ability of the device to convert the incoming radiation into an electrical signal. Detector material properties influence this value; therefore, several main material properties are investigated which include temperature coefficient of resistance (TCR), optical bandgap, transmittance, reflectance and absorptance and resistivity in the wavelengths of interest. Other properties such as compatibility with complementary metal oxide semiconductor (CMOS) processing technology, low cost and reliability and stability of the material while exposed to infrared radiation are important.
The microbolometer's sensing materials are classified in two main categories—metals and semiconductors. Metals such as Ti, Ni or Ni—Fe alloys had been reported as bolometer's sensing layers.
Amorphous Si (a-Si) and Vanadium Oxide (VOx) are two of the most widely used materials for sensing layers of microbolometers These two materials suffer from low TCR and low absorption which yields lower figures of merits such as responsivity, detectivity, and noise equivalent temperature difference. By using various atomic compositions of Germanium, Silicon, Tin and Oxide in Ge—Si—Sn—O thin films, this invention reports using Ge—Si—Sn—O thin films for microbolometer's sensing layer.
For microbolometers made of semiconducting sensing layers (semiconducting microbolometers), thermal change on a material with a high TCR causes a change in electrical resistance, thus allowing a measurable parameter across the detector with heating and cooling. Once the microbolometer's sensing layer's temperature changes, there are three possible mechanisms of heat loss. First, heat is lost through conduction/convection through the atmosphere surrounding the detector thermometer, that is described below. This is minimized by vacuum packaging the detectors. Today, it has been a common practice to include the wafer-level vacuum packaging scheme for all commercially available microbolometers. Second, heat is lost through radiation. The selection of materials will impact this mechanism. However, the materials that are preferred for low heat loss also absorb less infrared radiation, which is not desirable. This mechanism represents the ultimate limit on the performance of the detector. Third, the heat is lost through thermal conduction through the supporting structure of the thermometer. The design of the supporting structure will minimize the thermal conductance of the structure.
The performance of an IR detector for imaging is most commonly described by a parameter called noise-equivalent temperature difference (NETD), which is the measurement of how well a thermal detector will distinguish between slight differences in thermal radiation in the image. The best cryogenically cooled quantum devices will have NETD values below 20 mK. Although no thermal uncooled detector has reached such low values, the theoretical limits of thermal IR detectors operating at ambient temperature are close to the values of cooled quantum detectors for wavelengths above 8 μm. Since thermal noise power increases as the square root of heat conduction, the heat conduction to the environment poses the largest limit in terms of detection. To lower this limit, the thermal bridges between detector to substrate and housing must be minimized. Also, lowering the heat capacity of the detector element by reducing the thickness of the detector structure leads to a large temperature change per radiation input, further reducing the effect of noise. Commercial microbolometers with a lens of an f-number equal to 1 has an NETD value of 35 mK. Micromachining has allowed further improvement of thermal detectors, with the most advanced IR Focal Plane Array (FPA) currently based on microbolometers of vanadium oxide and amorphous silicon, achieving NETD between 25 and 50 mK.
For a thermal detector, the sensitive element is referred to as the thermometer. The thermometer is typically thermally isolated from the substrate to improve the responsivity by suspending it above the substrate using micromachining techniques. The performance of a thermal detector depends upon the thermal capacity Cth, the rate at which thermal energy is lost through the thermal conductance of the structure, Gth, and the radiative thermal conductance, Grad. The radiative thermal conductance for a gray body, assuming the emissivity is equal to the absorptivity, is given by equation Grad=4ησAT3; where η is the average absorption of the detector, σ is the Stefan-Boltzmann constant, A is the surface area and T is the absolute temperature. The conductive/convective loss is neglected since the detector is typically operated in vacuum. The temperature change due to a sinusoidally modulated photon flux is given by:
                              Δ          ⁢                                          ⁢          T                =                              η            ⁢                                                  ⁢            Φ                                                              G                eff                            ⁡                              (                                  1                  +                                                            ω                      2                                        ⁢                                          τ                      th                      2                                                                      )                                                    1              /              2                                                          (        1        )            
Where, Φ is the radiant energy flux, is the angular modulation frequency of the incident radiation, and is the thermal time constant (Cth/Gth). The effective thermal conductance, Geff, is obtained through a heat balance and is given by:Geff=Gth+Grad±αPbias  (2)
Where, α is the TCR of the thermometer, Pbias is the power dissipated in the bias of the detector. The sign of the power bias term depends upon the type of bias. The “+” sign corresponds to the voltage bias case while the “−” sign corresponds to the current bias case. For the case of a semiconductive microbolometer, the TCR is negative, which means that the power dissipated in the detector effectively increases the effective thermal conductance Geff.
There are other figures of merits than NETD for microbolometers which are described below:
Temperature Coefficient of Resistance (TCR):
TCR exhibits how rapidly the resistance of the sensing material responds to a change in temperature and is expressed as
                    α        =                                            1              R                        ·                          dR              dT                                =                                                    1                R                            ⁢                                                Δ                  ⁢                                                                          ⁢                  R                                                  Δ                  ⁢                                                                          ⁢                  T                                                      =                          -                                                E                  a                                                  kT                  2                                                                                        (        3        )            Here, Ea is the activation energy and k is the Boltzmann constant. TCR is a material property, so the higher the value, the better it is for IR uncooled detection.
Responsivity:
Responsivity is a measure of the dependence of the signal output of a detector upon the input radiant power. The detector output signal may be current or voltage. Thus the voltage responsivity, Rv, is defined as the detector output voltage per unit of detector input power.
                              R          v                =                              η            ⁢                                                  ⁢            α            ⁢                                                  ⁢                          RI              b                                                                          G                th                            ⁡                              (                                  1                  +                                                            ω                      2                                        ⁢                                          τ                      2                                                                      )                                                    1              /              2                                                          (        4        )            
Where, η, Ib, Gth, ω, and τ are the absorption coefficient, bias current, thermal conductance, angular frequency, and time constant of the device, respectively. The first three terms of the numerator in the right-hand side of equation (4) (η, α and R) depend on material properties of the microbolometer. Voltage responsivity is expressed in V/W while current responsivity is expressed in A/W. The voltage responsivity of the bolometer is increased by decreasing the thermal conductance of the structure. The thermal time constant of the microbolometer is in the millisecond range as it involves the thermal mass of the sensing layer which needs to heat up for change in resistance because of IR radiation.
Detectivity:
Detectivity, D*, is the area normalized signal to noise ratio. It has the unit of cmHz1/2/W. The detectivity is expressed by
                              D          *                =                                            R              v                        ⁢                                                            A                  d                                ⁢                Δ                ⁢                                                                  ⁢                f                                                          Δ            ⁢                                                  ⁢                          v              n                                                          (        5        )            where, Δvn is the total noise voltage observed in the electrical bandwidth Δf and is the sum of noises from the sensing element of the microbolometer—Johnson noise, random telegraph switching noise, 1/f-noise, generation and recombination noise. Higher responsivity represents higher detectivity.
Table 1 shows the list of materials used as the sensing layer of microbolometer infrared detector. The main drawback found in all the materials is the low TCR which also ends up in lower responsivity and detectivity when they are used in microbolomter.
TABLE ITCR and other figures of merit of bolometer sensing materials.TCRDetectivityResponsivityResistivityPixel sizeMaterial(%/K)(cmHz1/2/W)(V/W)(Ω-cm)(μm2)V2O52.8  6 × 105   361.7200 × 800V0.95W0.054.10  1 × 109N/A40N/Aa-SiGe−2N/AN/A~40N/APoly-SiGe−1.91 8.3 × 10815,000N/AN/Apoly-SiGe (CVD−22.31 × 1091.4 × 104N/A25 × 25deposited)a-Si:H2.8-3.9N/A  1 × 106N/A48 × 48a-GexSi1−xOy−2.27-−8.698.27 × 1061.05 × 104 4.22 × 102-3.47 ×40 × 40109Y—Ba—Cu—O4.02 1.6 × 1093.8 × 10532.42 7000 × 10000
In current invention, we observed that the absorption in the thin film was most sensitive in the wavelength ranges of 2.5-3.7 μm range. The optical energy band gap (0.22 eV) of the thin-film was using Tauc's equation. In addition to these, we also found the variations of absorption coefficient (6592305.87 m−1-11615736.95 m−1), refractive index (2.5-4.0), and the extinction coefficient (2.31-5.73) for the wavelength ranges between 2.5-5.5 μm. We found the thin film's Resistivity to be 142.55 Ω-cm by four point probe method.